A holographic recording and reproducing method is proposed as a recording technology that achieves a large recording capacity and enables high-speed transfer of information.
In the holographic recording and reproduction, a gas laser and a solid-state laser are usually used as a light source in order to keep wavelength stability and coherency. Those light sources have problems that they make the size of a device larger and increases a manufacturing cost of the device.
On the contrary, if a semiconductor laser is used as the light source, the size and the manufacturing cost of the device can be reduced. However, the semiconductor laser has a problem that it is inferior to the gas laser and the solid-state laser in the wavelength stability and the coherency.
In general, a recording layer of a holographic recording medium, in which a hologram is formed, has a thickness of several tens of microns or more, and preferably 100 μm or more, in order to increase recording density.
Such a thick hologram has angular selectivity and wavelength selectivity for a reproducing laser beam. That is, information can be reproduced only when a reference beam is incident on the hologram at an angle and a wavelength of a recording condition.
In general, multiplexed information is recorded in the same volume by changing an angle condition of the reference beam or the like in an appropriate manner.
When information is reproduced with a laser beam having a different wavelength from the wavelength of the reference beam corresponding to the recorded hologram, an intensity of reproduced beam (an intensity of diffracted beam) becomes weaker as compared with the case where reproduction is performed with a laser beam having a wavelength coincident with the wavelength of the reference beam. This is because there is a certain wavelength selectivity of the hologram. Moreover, angular distortion occurs in the reproduced beam. In this case, if two-dimensional data is recorded in the hologram, the two-dimensional data is reproduced with distortion.
The lowering of the intensity of the reproduced beam and the distortion in the reproduced beam described above can be corrected by design of a device for capturing two-dimensional data, such as a CCD or a CMOS, or a modulation pattern. However, correction is difficult when the wavelength difference is large.
The wavelength selectivity is enhanced in proportion to the thickness of the recording layer of a holographic recording medium. Thus, when the recording layer is made thicker in order to increase the recording density, even a slight wavelength difference between the reproducing beam and the reference beam makes reproduction of information difficult.
In order to overcome the above, Japanese Patent Laid-Open Publication No. 2002-216359 describes that a wavelength-variable semiconductor laser including an optical waveguide type wavelength conversion device is used as a light source to thereby vary a wavelength of a reproducing laser beam in accordance with thermal expansion and contraction of a recording medium. This is based on that a most appropriate wavelength of the reproducing beam is varied by thermal expansion and contraction of the recording medium during reproduction of information recorded by holographic recording.
Moreover, a holographic recording apparatus using a temperature adjustment element and a Fabry-Perot etalon in order to keep wavelength stability of a semiconductor laser during recording is proposed in Japanese Patent Laid-Open Publication No. Hei 8-202246. In this holographic recording apparatus, a temperature and an injection current of the semiconductor laser are made stable so as to make a wavelength of a laser beam during recording stable.
The aforementioned holographic optical information recoding and reproduction apparatus described in Japanese Patent Laid-Open Publication No. 2002-216359 has the following problem. In this apparatus, the wavelength-variable semiconductor laser as a wavelength-variable coherent light source and the optical waveguide type wavelength conversion device are controlled based on a state of a reproduced signal beam so as to make the wavelength of the reproducing beam most appropriate. Thus, it takes a long time to actually obtain a laser beam having the most appropriate wavelength after the reproduction apparatus starts up.
Moreover, there is another problem that it is difficult to obtain a short wavelength required for increasing the density in case of using the wavelength-variable semiconductor laser. In addition, the semiconductor laser generally has a variation of an oscillation peak wavelength between products having the same design and the oscillation wavelength varies with a temperature.
Furthermore, in the holographic recording apparatus described in Japanese Patent Laid-Open Publication No. 8-202246, the wavelength of a laser beam of a reproducing semiconductor laser during reproduction is not taken into consideration. Thus, reproduction may become difficult when an oscillation peak wavelength of the semiconductor laser in a reproduction apparatus is shifted from the wavelength of the laser beam used for recording because of an excessively high or low environmental temperature.